1,721,014 research outputs found
Stratigraphie und kalkige Nannofossilien des Unter-Apt von Alstätte NRW
No full textA new outcrop of Lower Cretaceous marine sediments (Lower Aptian) is described from Alstätte (western part of the Münsterland). The lithostratigraphy, the biostratigraphy, the calcareous nannofossil assemblages and the stable isotope signal (δ13Corg are documented. Based on a detailed lithological log the calcareous nannofossil biostratigraphy is being discussed. The studied succession can be attributed to the Flabellites oblongus nannofossil zone of early Aptian age
Early Cretaceous chalks from the North Sea giving evidence for global change
No full textAmong calcareous nannofossils, important primary producers in Jurassic and Cretaceous oceans, nannoconids were carbonate rock-forming organisms. During the late Barremian and early Aptian (∼126 to 122 million years ago), nannoconids went through a crisis culminating during the Oceanic Anoxic Event 1a. Here we present nannofossil and geochemical data from a section of early Barremian-early Aptian age from the North Sea, recording the earliest chalks ever known in the Boreal Realm. These middle-late Barremian chalks were generated by blooming of endemic nannoconids under relative warm and arid conditions. A subsequent decrease of nannoconids in the latest Barremian coincides with increased nutrient and clay input. This nannoconid decline, also detected at low latitudes, was associated with the Ontong Java Plateau emplacement. We conclude that nannoconids were rock forming also at high latitudes, under clear and oligotrophic waters. Their decline was related to increased continental runoff under reinforced greenhouse conditions
Integrated stratigraphy of Early Aptian black shales in the Boreal Realm: calcareous nannofossil and stable isotope evidence for global and regional processes
No full textThe Early Aptian is marked by an event of widespread anoxia in the oceans, known as Oceanic Anoxic Event (OAE)1a. During this time the Lower Saxony Basin (LSB), constituting the southern extension of the Boreal-Arctic Sea, was affected by the deposition of finely laminated black shales of the Fischschiefer (FS) considered to be the product of OAE 1a. This study focuses on Upper Barremian-Lower Aptian sediments from three different localities in northern Germany encompassing the FS. The proposed integrated litho-, bio-, and chemo-stratigraphy provides an accurate time control for correlation and for detecting the timing of the processes that affected the LSB during the deposition of the FS. The paleoecological and paleoclimatic reconstructions based on calcareous nannofossils indicate that sedimentation during the Late Barremian was mostly depending on regional conditions related to the paleogeography of the LSB. The deposition of the FS was instead mainly driven by mechanisms operating on a global scale and associated to OAE 1a: a warming event, also detected at low latitudes, was accompanied by high primary productivity and influx of cosmopolitan taxa through new seaways opened to the Tethys. In the late Early Aptian local factors prevailed again on sedimentation although paralleled by a decrease in temperature documented at different latitudes which probably favoured the migration of Boreal species southwards
Bio- and chemostratigraphy of the Posidonia Shale: a new database for the Toarcian Oceanic Anoxic Event from northern Germany
Full text linkWe present calcareous nannofossil biostratigraphy, calcium carbonate and organic carbon isotope data of two cores drilled in the North German Basin (northern Germany) covering the upper part of the Amaltheenton-Formation (Fm) (upper Pliensbachian) and the Posidonienschiefer-Fm. (Toarcian). Fourteen bioevents spanning the latest Pliensbachian to late Toarcian time interval allowed the identification of the NJ5, NJ6 and NJ7 Zones of the Boreal biozonation. The early Toarcian Oceanic Anoxic Event (T-OAE), identified by the organic carbon isotopic excursion within the Posidonienschiefer-Fm., is constrained by the first occurrences (FOs) of Carinolithus superbus crassus and Diductius constans at the onset of the δ13C anomaly. The last occurrences (LOs) of the nannofossil species Crepidolithus granulatus, Parhabdolithus liasicus distinctus, Biscutum finchii and Biscutum grande are detected within the δ13C isotopic anomaly. The new biostratigraphic data acquired in the North German Basin are compared to data from sections at higher and lower latitudes to evaluate event reproducibility relative to the δ13Corg isotope curve. The FO of C. superbus crassus is an excellent datum to constrain the onset of the T-OAE at supraregional – global scale. Our finding indicates further nannofossil biohorizons within the T-OAE that might be useful at regional scale
The Role Of Oceanic Plateau Volcanism On Climate Change: Warming And Cooling Episodes Across Early Aptian Oceanic Anoxic Event 1a
No full textThe early Aptian is marked by a global phenomenon of organic matter burial in oxygen-depleted oceans known as Oceanic Anoxic Event 1a (OAE 1a: ~120 Ma). Volcanism associated with the emplacement of the Ontong Java Plateau (OJP) is thought to be the main triggering mechanism for global anoxia, ocean acidification and greenhouse conditions. However, climate instability during OAE 1a is indicated by independent studies on TEX86, sporomorphs and oxygen-stable isotope but a direct connection between OJP volcanic phases and temperature variations has not been ascertained.
A high-resolution integrated nannofossil-geochemical investigation of distant sections from the Tethys, the Pacific Ocean and the Boreal Realm has revealed systematic and synchronous changes. Specifically, the nannofossil Temperature Index and Os-isotope records allowed the reconstruction of a complex series of global warming and cooling events across OAE 1a and their relationships with OJP volcanism as well as weathering patterns.
Two prominent volcanic phases are documented in the Os-isotope records: the first preceding OAE 1a and the second one, of major intensity, starting in the core of the negative C-isotopic anomaly. Both phases are paralleled by increased temperature, suggestive of a (super)greenhouse climate triggered by excess volcanogenic CO2. Indeed, our data indicate that the beginning of the prolonged volcanic phase during OAE 1a coincides with warmest temperatures.
In the early part of OAE 1a, between the two major volcanic phases, there is a ~100 kyrs-long interval characterized by a radiogenic Os-isotope peak, suggestive of accelerated continental weathering rates, with or without volcanism cessation, following an interval of abrupt warming and preceding a cooling interlude. Arguably, warming at OAE 1a onset promoted methane hydrate dissociation (also suggested by C-isotope and biomarkers analyses), which was perhaps instrumental in triggering continental weathering. Subsequent CO2 draw down, possibly during OJP quiescence, might explain the brief cooling interlude annihilated by warmest temperatures coeval with the onset of OJP paroxysmal phase.
In the second part of OAE 1a two more cooling events sandwich an interval of intermediate and fluctuating temperatures. The three cooling episodes correlate with high TOC content, suggesting that burial of organic matter acted as storage of excess CO2, thus temporarily mitigating greenhouse conditions, although under persisting OJP activity. The end of OAE 1a corresponds to the vanishing of OJP volcanism as recorded by Os-isotope. A major cooling episode decrees the conclusion of greenhouse conditions for the rest of the Aptian.
Increasing data and improved chronology show that volcanism of gigantic plateaus such as OJP is qualified to cause severe global warming and also indirectly to impact temperature changes. In fact, positive and negative feedbacks vicariously governed by prolonged (and possibly pulsing) formation of oceanic plateaus may be likewise or even more influential in controlling climate variability
Size variations of coccoliths in Cretaceous oceans : a result of preservation, genetics and ecology?
No full textBiometric studies of coccoliths, the remains of coccolithophores, offer the opportunity to survey
single species instead of entire assemblages. We obtained and analyzed size data of three
common species (Biscutum constans, Zeugrhabdotus erectus and Watznaueria barnesiae) in a
stratigraphically very well-defined interval of early Aptian age (~126 Ma; Cretaceous). Material is
derived from four sites (Lower Saxony Basin, North Sea, western Tethys, Mid-Pacific) covering
nearshore to open-oceanic paleosettings.
Length and width measurements of 1986 specimens were evaluated. The recorded size
patterns show a larger data spread for B. constans and W. barnesiae in the western Tethys and
the Mid-Pacific than in the North Sea and the Lower Saxony Basin. The latter two sites are
dominated by small coccoliths of B. constans while coccoliths of W. barnesiae show similar sizes
at all four sites. Solely small specimens of Z. erectus characterize the samples from the North Sea
and the Lower Saxony Basin while only large ones are present in the samples of the western
Tethys and Mid-Pacific.
For explaining the recorded size patterns, three theories are discussed in detail; these include
(1) preservation of nannofossils, (2) genetics and (3) palecology. (1) Intense dissolution or
overgrowth of the nannofossils may have altered the original coccolith sizes particularly when
biometric data from different sites with potentially varying states of preservation are compared.Due to its delicate morphology, Z. erectus appears most prone to dissolution, probably explaining
its size pattern. (2) If the recorded size data of the remaining two species represent original
patterns, these can be interpreted by new findings in recent coccolithophore genetics. It has been
shown that size variations within a single cryptic species are only minor. Shifts of coccolith sizes,
both in recent and fossil taxa, can be related to genotypic variation. The varying size ranges
recorded for B. constans and W. barnesiae may therefore reflect diversity changes of cryptic
species at the different sites. (3) These cryptic species may have preferred different depth habitats
depending on geographically-controlled factors such as sea-surface temperatures, light availability
or trophic load
Early Aptian black-shales in the Boreal Realm: calcareous nannofossil and stable isotope evidence for regional and global processes
No full textThe early Aptian is marked by a global phenomenon of widespread deposition of organic carbon-rich sediments under oxygen-poor conditions known as Oceanic Anoxic Event 1a (OAE 1a: ~120 Ma). Triggering mechanisms for the OAE 1a are thought to have been the emplacement of the Ontong Java Plateau, associated with global warming and enhanced primary productivity.
In northern Germany the OAE 1a has its sedimentary expression in the “Fischschiefer” characterized by finely laminated black shales rich in organic matter. In the Early Cretaceous northern Germany was part of the Lower Saxony Basin being a marginal epicontinental sea forming the southern extension of the Boreal-Arctic Sea, thus located between the Boreal Realm in the north and the Tethys in the south. The relatively restricted palaeogeographic settings probably exerted a regional control on the deposition of the Fischschiefer.
We investigated the Upper Barremian-Lower Aptian interval in three sites from northern Germany (Alstätte, Rethmar, Hoheneggelsen KB 9) applying a multidisciplinary approach, bringing together the biotic and ocean chemistry responses across OAE 1a. Specifically, we performed calcareous nannofossil (abundance and assemblage composition) and carbon and oxygen stable-isotope analyses.
Calcareous nannoplankton is sensitive to changes in temperature, fertility and chemistry of surface-waters, therefore it is the ideal tracer for reconstructing palaeoclimatic and palaeoceanographic fluctuations. The direct calibration of the nannofossil and geochemical data is fundamental to provide a coherent reconstruction of the perturbations across OAE 1a. The main objective of this study is to reconstruct palaeoclimatic, palaeoecological and palaeoceanographic factors that controlled the deposition of the Fischschiefer in the Lower Saxony Basin.
The results obtained for the Boreal Realm are compared and integrated with biotic and geochemical proxies collected across the OAE 1a from the Tethys (Cismon, N. Italy), with the main purpose of understating the interaction between globally driven changes in the ocean-atmosphere system and regional phenomena related to local palaeoceanographic settings
Early Cretaceous chalks from the North Sea giving evidence for global change
No full textIn northern Europe and elsewhere chalks are widespread throughout the Cenomanian – Maastrichtian interval. Less well known are, however, early Cretaceous chalks which have been encountered in the Central Graben of the North Sea. These chalks (Hauterivian, Barremian; Tuxen Formation) and the overlying mudstones of the Sola Formation (late Barremian to Aptian) have been studied by us (calcareous nannofossils, geochemistry) in order to better understand the palaeoceanographic setting of the Barremian – Aptian interval in the Boreal Realm.
The Barremian chalks were generated by the blooming of endemic nannoconids under relative warm and arid conditions. Among calcareous nannofossils, important primary producers in Jurassic and Cretaceous oceans, nannoconids were carbonate rock-forming organisms. In the Tethys nannoconids went through a crisis during the late Barremian and early Aptian, culminating during the Oceanic Anoxic Event 1a. This decline and subsequent crisis of nannoconids is now seen in the North Sea cores, thereby recording this signal also from the Boreal Realm. The decline of nannoconids in the latest Barremian coincides with increased nutrient and clay input. The nannoconid decline, also detected at low latitudes, was associated with the Ontong Java Plateau emplacement. We conclude that nannoconids were rock forming also at high latitudes, under clear and oligotrophic waters. Their decline was related to increased continental runoff under reinforced greenhouse conditions
Evolution of a Maastrichtian–Paleocene tropical shallow-water carbonate platform (Qalhat, NE Oman)
No full textThe biostratigraphy (larger foraminifers, dasycladaleans), microfacies, sedimentology, and geochemistry (d13C, strontium-isotope stratigraphy) of a continuous, 148- m-thick section of shallow-water platform carbonates that contain the Cretaceous/Paleogene (K/P) boundary were analyzed. The boundary is constrained within a 7-m-thick interval, between the last occurrence of Maastrichtian larger benthic foraminifers and the Wrst occurrence of Danian benthic foraminifers. Although this interval is intensively dolomitized, there is no sedimentological evidence of a major hiatus at the K/P boundary. The correlation of bulk rock d13C values with stable isotope data from DSDP Site 384 (NW Atlantic Ocean) supports this interpretation and indicates a Selandian age for the top of the section. The Qalhat section is a unique example of a carbonate platform that has recorded persisting open marine environmental conditions across the K/P boundary (Maastrichtian– Selandian), as indicated by the abundance of rudists, larger benthic foraminifers (Maastrichtian), calcareous algae and scleractinian corals
Evidence for high sea-surface temperatures during the early Aptian OAE 1a in the Boreal Realm
No full textThe early Aptian was characterized by the
widespread occurrence of anoxia in the oceans,
known as Oceanic Anoxic Event (OAE) 1a.
Intense degassing from submarine volcanic
plateaus presumably resulted in high
atmospheric CO2 concentrations, culminating
in greenhouse conditions (e.g., Hochuli et al.,
1999; Jenkyns, 2003; Erba et al., 2010). OAE
1a can, therefore, be considered as a past
“natural experiment” important to understand
the evolution of our future climate.
Paleotemperature estimates for OAE 1a are,
however, predominantly based on bulk
oxygen-isotopes, which are susceptible for
diagenetic overprinting, while TEX86
paleotemperature estimates are limited in
number or derived from stratigraphically
poorly constrained sections.
We reconstructed for the first time sea-surface
water (SST) temperatures based on the TEX86
paleothermometer from an OAE 1a section
from the middle northern latitudes (39° N
paleolatitude). We find a SST rise starting
prior to OAE 1a and reaching a maximum
during the event with SSTs around 31–34 °C,
4–9 °C higher than those of older Hauterivian -
lower Aptian sediments from the same
sedimentary basin (Mutterlose et al., 2014).
The end of OAE 1a is marked by relatively
lower SSTs around 30 °C. These observations
are supported by belemnite-based oxygen
isotope data and calcareous nannofossils. Our
integrated data set clearly indicates that ‘super
greenhouse’ conditions prevailed during OAE
1a at northern latitudes. SSTs are similar to
those estimated for coeval low latitudinal sites,
suggesting that an equable warm climate, with
reduced latitudinal gradients, characterized the
early Aptian.
References
Erba, E., Bottini, C., Weissert, J.H. & Keller, C.E.
(2010): Calcareous nannoplankton response to
surface-water acidifi cation around Oceanic
Anoxic Event 1a: Science, v. 329, p. 428–432.
Hochuli, P.A., Menegatti, A.P., Weissert, H., Riva,
A. Erba, E. & Premoli Silva, I. (1999): Episodes
of high productivity and cooling in the early
Aptian Alpine Tethys: Geology, v. 27, p. 657–
660.
Jenkyns, H.C. (2003): Evidence for rapid climate
change in the Mesozoic–Palaeogene greenhouse
world: Philosophical Transactions of the Royal
Society of London, Series A, v. 361, p. 1885–
1916.
Mutterlose, J., Bottini, C., Schouten, S. &
Sinninghe Damsté, J.S. (2014) : High sea-surface
temperatures during the early Aptian Oceanic
Anoxic Event 1a in th
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